1. Field of the Invention
The invention pertains to a circuit for controlling a double-action hydraulic drive cylinder having a piston separating a piston space from a rod space, wherein hydraulic oil can be supplied to the piston space while hydraulic oil simultaneously flows out of the rod space, and hydraulic oil can be supplied to the rod space while hydraulic oil simultaneously flows out of the piston space, the cylinder being actuated by a directional control valve having working ports A and B, a pump port P, and a tank port T.
2. Description of the Related Art
Double-action drive cylinders are often used in devices for raising and lowering loads. In the one direction of movement, hydraulic oil is fed into the piston space of the drive cylinder, whereas hydraulic oil must be discharged from the rod space of the drive cylinder. Because the cross sections of the piston and rod spaces are different, the quantities of hydraulic oil fed in and discharged are also different. In the first direction of movement just mentioned, the amount of hydraulic oil which must be fed into the piston space is greater than that which flows out of the rod space. The situation is reversed for the other direction of movement.
If the inflow and outflow of hydraulic oil is controlled by a directional control valve, all of the hydraulic oil to be supplied to the piston space, for example, must be conveyed by a pump. The hydraulic oil flowing out of the rod space flows to the tank by way of the directional control valve.
A differential circuit is known from the publication Der Hydraulik Trainer (The Hydraulic Trainer), Vol. 2, Proportional and Servo Valve Engineering (Mannesmann Rexroth GmbH, 1st edition, ISBN 3-8023-0898-0). In this circuit, a spring-loaded check valve is installed in parallel to the directional control valve. When the pump conveys hydraulic oil via the directional control valve to the piston space, hydraulic oil flows out from the rod space via the check valve to the pump port of the directional control valve, because the return flow to the tank is blocked by the directional control valve. The pump must therefore convey only the differential quantity of hydraulic oil.
In the case of work machines in which such double-action drive cylinders are used, the pipelines between the directional control valve and the double-action drive cylinder are often very long, such as 8 meters or more. A long hydraulic oil line, however, represents a hydraulic resistance, which translates to energy losses and to the heating of the hydraulic oil.
Another circuit is known from U.S. Pat. No. 5,826,486. A circuit is designed here with a check valve between the feed line leading to the rod space and the feed line leading to the piston space. Thus hydraulic oil can flow from the rod space to the piston space without having to detour by way of the directional control valve. This at least partially solves the problems of energy losses and oil heating. So-called regeneration is therefore active when the rod travels out of the drive cylinder, which can mean, for example, that the load is being raised. When the rod travels inward, that is, when the load is being lowered, for example, no regeneration takes place. The entire quantity of hydraulic oil leaving the piston space of the hydraulic drive cylinder must be carried away to the tank via the directional control valve, whereas the quantity of hydraulic oil to be conveyed into the rod space must flow from the pump via the directional control valve. When the load is lowered, the pump must therefore provide power, and the entire quantity of hydraulic oil must flow through the long lines.
A controlled suspension circuit for an actuating device is known from U.S. Pat. No. 6,092,454. Here regeneration from the piston space to the rod space of a hydraulic drive cylinder is possible, but it requires additional control means, namely, a pilot-controlled check valve, which is actuated by an electrically controlled valve. The electrically controlled valve for its own part is actuated by a contact of a switch arrangement. In one of the embodiments, furthermore, a second pilot-controlled check valve is necessary, which is controlled by a proportional pressure control section. In the second embodiment shown, an additional outlet valve is required, which must be actuated by a second proportional pressure control section.
Regeneration of the piston space to the rod space is therefore possible in principle here, but it requires control measures and is tied to the presence of pilot-controlled check valves and their actuating elements. Hydraulically controlled valves and their actuating elements, which also act hydraulically, lead to pressure losses and thus impose a certain power demand.